1. Field of the Invention
The present invention relates to shaping glass sheets, and particularly, refers to the shaping of glass sheets into complicated shapes incorporating a sharp line of bending extending completely across a dimension, more specifically the width, of the bent glass sheet. Such a line of bending provides a continuation of a line of sharp bending in the automobile body in which the bent glass is installed. Such bends satisfy the desires of automobile stylists to incorporate shaped glass sheets whose shapes conform to the shape of the automobile body in which the glass is mounted. Such bends are useful as windshields, back windows and roof windows of automobiles and other vehicles and may also be used in shaped windows in buildings.
2. Description of the Prior Art.
In recent years, several patents have issued relating to the bending of glass sheets to sharp curvatures. U.S. Pat. No. 3,762,903 to Harold E. Hamilton discloses the application of an elongated line of electroconductive frit of ceramic silver or other suitable electroconductive material in one or more layers along a line desired for sharp bending to one or more surfaces of one or more glass sheets to be shaped on an outline bending mold defining a V-shaped configuration desired for the bent glass. The glass sheet is initially heated to a temperature slightly below the glass softening point that is sufficient to fuse the frit along the line of sharp bending. Electrodes are held against the opposite ends of the electroconductive frit and voltage is impressed between the electrodes. The impressed voltage passes current through the frit, thereby heating the frit electrically. The electrically induced heat raises the glass temperature locally along the line desired for sharp bending. The glass sags rapidly along the line of sharp bending until it conforms to the shaping surface of the V-shaped outline mold. The frit remains as an opaque line extending across a dimension of the ultimate product because it is very difficult to remove a fused ceramic frit.
U.S. Pat. No. 3,762,904 to Harold E. Hamilton, Robert E. Bamford and Paul Pastorek discloses forming a groove in a glass surface along a line of sharp bending, applying a line of electroconductive frit that is fusible onto the glass either in the groove or aligned with the groove along the surface of the glass opposite the grooved surface. The electroconductive frit fuses to the glass to provide an opaque line in the finished product. Furthermore, while grooves accentuate the rate at which the glass sheet sags along the line of sharp bending, grooving weakens the glass and also enhances optical distortion. Therefore, grooving preferably should be avoided.
U.S. Pat. No. 3,879,184 to Harold E. Hamilton and Ivan L. Soreghy discloses a process for altering the residual strain remaining along the relatively sharp bend of a glass sheet resulting from the passage of an electric current through an elongated line of electroconductive material by interposing a strip of coloring agent between the glass sheet surface and the electroconductive material or line lengthwise of the line before applying the voltage that causes electrical heating of the glass along the line desired for sharp bending.
In each of the three patents enumerated thus far, the application of electrodes to the ends of the lines of electroconductive material causes problems in maintaining the sharp bend at the ends of the line of sharp bending. U.S. Pat. No. 3,865,680 to Thomas J. Reese and Harry S. Koontz discloses a technique where transverse extensions of the line of electroconductive frit are applied to extend in offset relation from the ends of the line of sharp bending to provide relatively large areas for electroconductive contact with the electrodes through which electrical power is applied. While this invention improves the sharpness of bend, particularly at the edges of the bent glass sheet, this technique also leaves the problem of removing frit from the bent glass sheet.
U.S. Pat. No. 3,795,501 to Robert A. Jansson and Thomas J. Reese, U.S. Pat. No. 3,795,570 to Robert A. Jansson and Dean L. Thomas and U.S. Pat. No. 3,847,586 to Thomas J. Reese, George R. Claassen and Melvin W. Tobin relate to method and apparatus for bending glass sheets into curvatures including a sharp bend using one or more narrow, elongated ribbons of electroconductive material which may be removed from the line of sharp bending after the glass is shaped. In each of the latter three patents, it is necessary to complicate the loading and unloading of the glass relative to the mold because of the need to apply and remove the narrow, elongated ribbon of electroconductive material relative to the glass in conjunction with loading or unloading the glass relative to the mold.
U.S. Pat. No. 2,111,392 to Henry J. Galey, U.S. Pat. No. 2,176,999 to Robert A. Miller, U.S. Pat. No. 2,215,228 to James G. Oliver and U.S. Pat. No. 3,248,195 to James S. Golightly and Harold E. McKelvey disclose the use of radiant heaters in glass sheet bending apparatus that apply sharp localized bends to a glass sheet along an enlongated axis thereof. However, these patents show apparatus that support the heaters in such positions that it is difficult to load or unload a glass sheet from the mold.
U.S. Pat. No. 2,999,338 to Ronald L. Richardson discloses a mold for bending glass sheets to compound curvatures. In this patented apparatus, electrical heating elements are carried by a pivotable overhead member which must be pivoted out of the way to load and unload a glass sheet relative to the mold. In addition, localized heaters are permanently installed within the outline end sections of a sectionalized mold. These localized electrical heaters are permanently mounted below the glass surface in the open or spread-apart position of the end mold sections of the sectionalized mold and the glass moves upward away from the localized electrical heaters as the glass is lifted while it is shaped.